Railroad grade crossing protection system



April 12, 1966 c. M. STEELE ETAL 3,246,143

RAILROAD GRADE CROSSING PROTECTION SYSTEM ZEO April 12, 1966 c. M. STEELE ETAL 3,246,143

RAILROAD GRADE CROSSING PROTECTION SYSTEM 7 Sheets-Sheet 2 Filed Sept. 30, 1963 April 12, 1966 c. M. STEELE ETAL 3,245,143

RAILROAD GRADE CROSSING PROTECTION SYSTEM APY!l 12, 1966 c. M. STEELE ETAL 3,246,143

RAILROAD GRADE CROSSING PROTECTION SYSTEM Find sept. so, 1965 7 Sheets-Sheet 4 o kon@ April l2, 1966 c. M. STEELE ETAL 3,246,143

RAILROAD GRADE CROSSING PROTECTION SYSTEM Fildd Sept. 30, 1965 7 Sheets-Sheet 5 rma April 12, 1966 c. M. s'rEELE ETAL 3,246,143

RAILROAD GRADE CROSSING PROTECTION SYSTEM '7 Sheets-Sheet 6 Filed Sept. 30, 1963 April 12, 1966 c. M. STEELE ETAL 3,246,143

RAILROAD GRADE CROSSING PROTECTION SYSTEM med sept. 3o, 196s '7 Sheets-Sheet 7 United States Patent O 3,246,143 RAILRAD GRADE CROSSING PROTECTION SYSTEM Carroll M. Steele, Los Altos, and Arlo C. Krout, San Carlos, Calif., assignors to Southern Pacic Company, San Francisco, Calif., a corporation of Delaware Filed Sept. 30, 1963, Ser. No. 314,578 17 Claims. (Cl. 246-128) This invention relates to systems for providing a warning at a railroad grade crossing when a train is approaching and, more particularly, to improvements therein. This application is a continuation-in-part of our application Serial No. 19,747, iiled April 4, 1960 and entitled Railroad Grade-Crossing Protection System. P-resently employed grade-crossing warning systems, although reliable, are fairly costly to install and maintain.

An object of this invention is to provide a novel railroad grade-crossing warning `sys-tem whereby the delay to cross trac is minimized.

Yet another object ofthe present invention is the provision of a unique railroad grade-crossing warning sys- 'tem which is easy to install and maintain.

Still -another obj-ect of the presen-t invention is the provi-sion of a novel railroad grade-crossing Warning system which is reliable.y

These and `other objects of the present invention are achieved in an arrangement wherein the railroad track is considered as a shorted transmission line in which the short is provided Iby the train. An alternating current signal with a substantially constant current level is applied to the tracks at the location of the grade crossing. The voltage existing across the tracks as the train, and therefore the short, approaches the grade crossing will diminish. Thu-s the amplitude of this voltage provides a measure of the distance of the train from the track while the rate at which this voltage diminishes provides a measure of the velocity of the train. With these parameters, it becomes possible to estimate the time of lthe trains arrival at the crossing. Knowing the time of arrival, the system can start warning signals at such a time as will provide the least possible delay for cross traic. The signal representative of distance and the derived signal therefrom representative of velocity are combined to provide a third voltage representative of the 'time required for the train to arrive at the railroad grade crossing.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a block diagram of an embodiment of the invention.

FIGURE .2 is a block diagram of another embodiment of the invention.

FIGURE 3 is a block diagram of yet another embodiment of the invention.

FIGURE 4 is a lblock diagram showing how the embodiment of the invention is connected to several successive track sections.

FIGURES 5A, 5B are detailed block diagrams of the arrangement of FIGURE 4.

FIGURE 6 is a block diagram of a modification of the embodiment shown in FIGURE 3.

IFIGURE 7 is a detailed yblock diagram of a portion of FIGURE 6; and

FIGURE 8 shows the waveforms of various signals present in FIGURES 6 and 7.

Reference will now be made to FIGURE l of the drawing which is a block .diagram of one Aembodiment of this invention. From FIGURE l it will be seen that a trainhas a motion in the direction Arepresented on a pair of track rails 12A, 12B. The .train .is at vsome distance L from an origin pointlP, vP', Which represents the location of a grade crossing,'for example. The vtrain motion occurs from left to right. The velocity V and acceleration A factors are therefore represented as going from left to right Aon the drawing. Now

L=ff(aodf)di (1f) and integrating,

L=f(10fi-Co)df (2) C0 is the integration constant.

The ,first integral of acceleration is velocity; there- The second integral of acceleration yis displacement. Therefore t2 =+va+co 7) Now at t: T0=0, we will let'the displacement =0. Hence 2 igt-+V@ (s) If we put limits on time inthe above equation, wezwill arrive at the distance an lobject Vwill travel in the time interval defined by these limits, as follows:

2 T (T2-TD2) L To; T1= qid-vllt] xzkLkVdTr-Tu) 2 fr,l 2

(9) Since T0=0, the above reduces to,

f 2 L T,T,=a2T +VT (10) where T1 is replaced by the general term T. Therefore, if a train has the distance L to go until it arrives at the crossing, it will take time T to get there. This is the basic equation of the prediction computer. In practice, L, a0, and V0 are functions of time, and the computer output is a running predicti-on of the time required for the train to get to the crossing. Because L, au, and V0 are varying with time, the prediction equation is written as A computer in accordance with this invention will include an oscillator 14 which oscillates at a suitable frequency. It has been found that from-the standpoint of obtaining best results an `oscillator which provides an output having a frequency on the-order of 50 cycles per second enables an optimum of results to be obtained. Higher frequencies give a higher input impedance, which is desirable, but bad track ballast. conditions load the transmission line, vwhich the railsrepresent, to such. an extent as to make a distance measurement with the required accuracy impossible. Frequencies below 50 cyclesy per secistant `current generator.` .track at substantially a constant current.

`ond makes the loading effect less severe, but they-reduce the input impedance, thus making -the circuit constants and voltages more difficult `to handle. The output of the oscillator is applied to excite a power amplifier 16.V A

resistor 18 connects one-side of the power amplifier to `one rail at a point'P. l amplifier is connected to the other rail at P. The power .amplifier 16 4together with resistor 18 comprises a con- This delivers Yan inputto the The other side of the power It should be appreciated that as the train approaches the points P, PY on the `tracks to which current from the constant current generator is applied, the impedance of the tracks looking towardthe train from those points is Icontinuously being diminished. This follows by reason of the fact that theftrain comprises a short across the tracks which is moved toward the points P, P. With current beingV maintained constant, the voltage at the points P, P will continuously decrease to a minimum .l

-when thetrain reaches Vthe points P, P. Therefore, by

measuring the Yvoltage across vthe tracks an indication is obtained of the distance of the train from the points at "which the voltage is impressed. The change with respect to time of this voltage can provide` velocity information, and the second derivative of this voltage information provides information as to the acceleration ofthe train. f

Accordingly, a narrow bandpass filter centered at 50 cycles which is connected to thesame points on the track as the constant current generator receives a voltage representative of length of track L or distance between the train and the points P, P. This voltage is an alternating current one, which is modulated by the motion of the train towardthe points P, P. The output of the nar- 4row bandpass filter is applied to a rectifier 22, which demodulates this voltage and applies its output to a lowpass filter 24. Its output isa kvoltage representative of the distance L.

The amplifier output isj applied to a differentiator cirv `cuit 26. The output of the differentiator circuit 26 will .be a voltage representative of the velocity V of the train.

:This voltage is applied to a Vsecond differenti-atar lcircuit 28 and also to one input of a multiplier circuit 30. The

second ditferentiator circuit 28 differentiates its input with Y respect-to time-'and provides'as lan outputa voltage representative of the accelerationof the train. The output Aof the second differentiator circuit is applied to a multiplier circuit 32. Y Y

Now considering the outputof the amplifier 20 again,

vin addition to being applied 'to the differentiator circuit 26, it isalso applied to an inverter circuit 34.- The `inverter circuit merely phase inverts the input Vso that the4 v output represents the negative of the input, which here is -L.` The output of the inverter circuit 34 is applied 'to an amplifier 36, which multiplies its input by a factor of two; The output of this ZX-amplilier 36 is appliedto one input of a summing amplifier 38. The summing amtplifier 38 also receives the output of the multiplier circuit `32. The multiplier circuit 30 applies its output tol another 2 amplifier 40. This amplifier output is then applied as a third input tothe summing amplifier 38. The output of the summing amplifier 38 is applied to a highassumption, then the multiplier circuit 44 provides asan output the quantity t2, which is multiplied by the acceleration-representative voltage; therefore, the input to the summing amplifier from multiplier circuit 32 is m2. The summing amplifier adds these quantities and if the voltage t has the right value, its output should be Zero.' The purpose ofV the high-gain amplifier 42 is to provide an output which is fed back in the manner VVshown to force the vsumming amplifier. output toward Zero. With-.a sufficiently high 'gain for the amplifier, this condition is approached. The outputof the highgain amplifier 42 therefore will be lthe quantity t or the expected arrival time of the train :at the points P, P'. This quantity is applied to an amplitude comparator circuit 46. Thus, the voltage t is compared by this circuit with a voltage provided by a voltage source 48.

V',l'hervoltage provided by the voltage source 48 has an amplitude-representing'the timeat which it is desired vthat the warning device at the railroad grade crossing be 20A actu-ated. Thus,lthe amplitude comparator circuit provides an output'to a warning device 50 Whenever the voltage tr and thatof the voltage source are equal, or,

whereTr=predicted arrival time, L=distance from the 'crossing Vto .the trains, and dL/ dt=the speed of the train.

All that Equation 13 sets forth is the well known con- `cept that the product of the velocity, times the time, -minus the distance, equals zero.

FIGURE 2 is a block diagram of an arrangement in accordance with this invention forproviding the desired prediction. time and actuating a warning device when this predicted timeV approaches a desired value. The train 10 on rails 12A, 12B is approaching the points P, P'. These are excited by an oscillator 60 which has the desired frequency of cycles per second.` The oscil- V lator output is vapplied to a Vpower amplifier @62. This is gain amplifier 42. The output of this high-gain amplifier is fed back as the second input to a multiplier circuit `5 .30 and as both of the inputs of a multiplier circuit 44.

The `output of the multiplier circuit 44 is applied las' a second input to the multiplier circuit 32..v

The Yinput to the summing amplifier 38 from the amand another voltage which it will be assumed represents -the estimated time of arrival I of the train at points P, .-P', then the output of the multiplier circuit is the prod- `uct Vt, and the input to the summing amplifier from the `.2X-'amplifier 40 is a quantity 2Vt.

Using the previous plifier 36 is -2L. Assuming the inputs to multiplier Acircuit 30 to be the voltage-representative velocity V connected to the points P, P through a resistor 64. This resistor 64 together with the power amplifier 6,2, as before, constitutesia constant-'current generator. The timeof-arrival predictionsystem includes a bandpass ampli- `Vfier ycontaining two Vactive filters,V respectively 66, 68. These filters are resonant to 49 and 5l cycles per second,

respectively, and Vforma staggered tuned'pair, which is four ycycles at the 3-,db points. The purpose of this wfilter is to remove noise vsignalsfrom-the systemv which do not lie close to the frequency of 50 cyclesy per second. 1 The output Vof this bandpass amplifier is applied to a rectifier and filter circuit for the purpose of removing the 50-cycles-per-second carrier component and its harmonics from the detected. signal. The signal is "then applied yto an amplifier 72 to re-establishits level and then is applied lto a cathode follower V'74,for the purpose of lowering the source impedance before coupling it with the remainder of the circuits. At this point,

Hthe signal is a changing direct current voltage, proporltional to the distance between the crossing points P, P' l andthe train.

The outputof the cathode follower is applied to a` minimum-distance amplitude discriminator ,76 whichwill be discussed later herein, as well as to a differentiator circuit 78.v 4The differentiated signal which represents the velocity V.is1 amp'lified'and inverted by Aa D.C. amplifier-80. The output of the D.C. amplifier 8f) is coupled to onerinput of av multiplier circuit.82. The output of the multipliercircuit'is connectedto a lD.C. amplifier 84, which serves thev function of amplifying and inverting its input. The output of the D.C. amplifier 84 is applied to one input of a summing amplifier 86.

The output of the cathode follower 74 is also applied to this summing amplifier 36 as its second input. The output of the summing amplifier is applied to another D.C. amplifier 88, the output :of which is applied to au amplitude discriminator 90 as well as to .the multiplier circuit 82 as its second input. The amplitude discriminator 90 controls a relay amplifier 92, which yoperates a relay 94 connected to the warning device when the time predicted by the circuitry described equals the time set in the amplitude discriminator.

Operation of the computation feedback loop within the prediction system is as follows:

The equation for which a solution is desired is L=the instantaneous distance between the train and the crossing, -which is obtained at the output of cathode follower 74;

T= the instantaneous transit time required for the train to travel from its present position to the crossing, obtained at the output of D.C. amplifier 881;

dL/ dt=the instaneous speed of the train;

and the means by which it is solved are given in the following steps:

(l) A solution, Ta, is assumed, and the solution is then multiplied by the speed, LiL/dt, by the multiplier circuit 82 with the following product resulting:

TaXdL/dt (15) (2) This product is subtracted from L in the summing amplifier I86, and the difference between the two quantities is denoted as k(t), then (3) k(t) is greatly increased in magnitude by passing it through an amplifier 88 having a gain A. The output .of this amplifier 88 is therefore A[k(t)]=A[L-Ta(dL/dt)] (4) the quantity YA[L--fITa-(dL/dO] (18) is entered into the feedback loop as the assumed solution, Ta. Therefore Ta=A [L-T(dL/dt)] (19.)

-or I Ta/A =L-Ta(dL/dt) (20) Therefore, if A T,

oL-:rgdL/df) (2.1)

90 produces an output-which is amplified and then operates the relay 94 connecting power to the waming signal. The voltage proportional to thefdistance to the train also Ais connected to the minimum amplitude discriminator 76.

This discriminator operates a relay 96 when the distance to the train is less than some preset value (i.e., 100 feet). This relay circuit acts as an override in the warning system and is used to overcome some of the prediction inaccuracies which occur when the train is moving very slowly.

Further'simplification of the embodiment ofthe invention shown in FIGURE 2 may be achieved, `if desired. Consider the block diagram of this embodiment of the invention, shown in FIGURE 3. A SO-cycle oscillator drives a power amplifier 102. The output of lthis amplifier is applied to the rails 12A, 12B to point P directly and'to point P through a resistor 103. As before, the amplifier 102 and resisto-r 103 comprise a constant current generator. These points are also connected to bandpass amplifier 104 of the type described previously, whichincludes two active filters which are resonant at 49 and 5l cycles per second. The output of the bandpassamplifier 104 is applied to a rectifier 106. The output of therectifier 106 is applied to a filter 108, the output from which is applied to an inverting difierentiator circuit 110 and also to a summing amplifier 112. The output of the inverting differentiator circuit is applied vto an amplifier 116. The output of amplifier 116 is connected to the second input of the summing amplifier 112. The summing amplifier output is connected to a high-gain amplifier 118. The output of the high-gain amplifier is applied to an amplitude comparator 120.

Voltage from a reference source 122 is also applied 'to the amplitude comparator to be compared with the out- `put from the high-gain amplifier 1.18. The output of the amplitude comparator is applied to `a relay amplifier 1213, Iwhich operates the warning signal relay 124' when the signal applied thereto has a sufficient amplitude. An override circuit is also provided, and this includes an amplitude discriminator 130, which receives output'from the rectifier 106, and also to a relay .amplifier 132 which .drives a relay 134. The relay amplifier 132 receives out'- put from the amplitude .discriminator 130. The signal which is provided at the output of the filter 108 is a varying direct current voltage proportional to thedistancc L between the :train andthe excitation point. This signalis then differentiated, inverted, and amplified by thefollowing circuitry to give a signal proportional to speed. This signal, which is added to the distance signal L by the summing amplifier, provides a result at the outputrof the-summing amplifier which can 'best be expressed in `an equa- .tion of the following form:

L-K(lIL/dt=k(t) (22) If it is desired to have a warning time of 30 seconds, then the gain of the amplifier following lthedifferentiator is set to 30 and the following equation results:

L-30(dL/dt) =k(t) (23) Consider now a train along distance off, so that L is larger than 30 dL/dt; under these conditions, k(t) will be positive. If, however, the trainproceeds at constant speed, there will be a time when L equals 30 dL/dt and.k(l) will be zero. A short time later L will be less than 30 'dL/dt and k(t) will be negative. When k(t) is zero, it will be exactly thirty seconds until the train arrives at the lexcitation point lif it does not change its speed. Therefore, all that is now required of the computer is tofdetect the `zero-crossing point of k(t). This is done by greatly amplifying k(t) and using an amplitude comparator to detect the zero-crossing. The output of the comparator is coupled to a relay amplifier which operates the warning relay.

This computer differs from the previous first-order computer in that an explicit solution for the warning time T is obtained only once, and that is when the warning time is equalto that which wasy preset into the computer. By using'this simplified predicter withoutthe inaccurate multiplier and its Aassociated feedback amplifiers, an increase in accuracy was obtained with no sacrifice insystem performance.

In order to incorporate the computer intoa warning system, lit is sometimes necessary to relay information. A simplified block diagram showing transmission of .information from one'crossing to another is shown in FIG- URE 4. The system can be described as follows: Assume -that there is a train in section No'. D2, proceeding at a velocity .V toward section No. D3. C22 are both tracking thistrain; however, the train is receding from crossing'X1, and vtherefore this barrier has been raised. Depending upon the train speed, several conditions of the barriers ahead of the train are possible: If the speed of the train is low, barrier X2 will lower only -when the train nearsrthe X2 crossing. In the same situation, barrier X3 will lower only when the train is nearing this crossing. In both cases, the computer is using only 'the information given to it from the track section in which -the train is located. However, if the train were .moving .at a high speed and if the Vtrack section No. D3 were short, ,it would be necessary for barrier X3 to come down while the trainis in section No. D2. To handle this type of a situation,-vv there is an additional circuit located in each com-puterto make the required computation in the follow- .ing manner: To the voltage in computer C22 (which represents the distance between the train and crossing X2), Sthere is added an additional fixed voltage proportional to -the distance D3.V The resulting sum is used` to compute arrival vtime at barrier X3. When the train has reached a point where barrier X3 should be lowered, a relay signal is .sent via the track signal line to computer C33, telling it to bring down the barrier.V When the train enters section No. 3, computer AC33 will be using information derived from `track section No. 3 and the relay information from C22 will be dropped out. can relay information to C41, etc., and for trains movin :in the other direction a similar situation exists.

Reference is now made to FIGURES 5A and 5B, which illustrate in more detail the simplified arrangement shown in FIGURE 4. Apparatus in FIGURE 5A providing -the same function as the apparatus shown in FIGURE .3 and FIGURE-4 will have the same reference numerals applied thereto. Vf Thus, the computers C11 and C12 in FIGURE 4., are shown in detail. These, in turn,' have the rcircuit shown in FIGURE 3. Computers C22 and -C23,.-as well as C33V and C34, which are identical with C11 and C12, are exemplified by the rectangular blocks. v An oscillator 100 oscillating at the preferred frequency of 50 cycles per second drives a power amplifier 102. The power amplifier excites thersections yof track desig- 'nated as D1 and D2 through a transformer 103. These track sections lie on either side of a crossing designated `as X1. The transformer 103 has two secondary windings 103A and 103B, which are respectively connected through resistors 105A, 105B toithe track` sections D1 and D2..

Batteries 120A, 120B and chokes 122A, 122B are respectively connected in series with each otherV `and then respectively across track sections D1 and D2. These batteries serve vthe function of providing the energy re-` Computers C12 and In a similar manner, computer C33 l Aside from a common oscillator and poweramplitier,

the remaining structure ofthe two computers, corresponding to C11 and C12 in FIGURE 4, are identical with that shown in FIGURE 3. Therefore, these will not be r'edescribed here. The outputs of the high-gain summing amplifiers 112a and 112b` are both applied to fiers'136A, 136B. 'I'he output of these ampliers is` respectively applied to :two rectifiers and filters 138A,

138B. The output from the rectifiers and filters is Vap- 'plied to Vtwo amplitude discrirninators, respectively 140A,

140B. A reference voltage is respectively applied to 4these respective amplitude discriminators fromsources 141A, 141B, which is indicated as the minimum-distance reference voltage. The purpose of this is that in the event the distan-ce of the train is less than this minimum distance in the respective blocks D1, D2, then an output will be applied from the respective amplitude discriminators to operate theV relay amplifier 123 and relay 124 to provide a warning.V These circuits are used to handle the situation where the train has come to a standstill, e.g., at a station near the railroad grade crossing, and it then moves slowly ytoward the railroad grade crossing. At thev minimum distance which has been established by the reference voltage, the railroad grade-crossing barrier, `or warning. device, will be operated whereas the predicted-time-of-arrival circuits would not satisfactorily operate the barrier. f

In addition to the circuits .shown for the two computers, at each barrier yor grade crossing there are also provided, respectively, two high-gain summing amplifiers 142A, 142B.4 These have three inputs applied thereto,

which'are added. Two of these inputs are identical with `those applied to the high-gain summing amplifiers 112A -and 112B andthe computer. representative of the distance of the train from the cross- These .are the signals ing and the Lsignal representative of the product -of the gain factor times the velocity. The third input to each one of the high-gain summing amplifiers has been designated as the distant-'crossing reference voltage derived in time at the -crossing X1.V Therefore, a train which is in the block section D1 and which is being tracked by "the Acomputer C11, when the sum Iof the three signals 'applied to the high-gain'surnming amplifier 142A attains a sufiicient amplitude, will operate the relay amplifier 146A, which will operate relay 148A.'V Contacts 148AC of this relay will be closed as the result. Closing of the Ycontacts 148AC enables the energization of a warning relay 150 which is positionedI at 4the next crossing X2.

The contacts C associated with this relay are connected in a circuit to energize theV grade-crossing warning device, or barrier. It should be noted that these contacts are connectedV and parallel with the contacts" 124C', which are associated with the time-of-arrival-prediction computer relay contacts. y

The high-gain summing amplifier 142C Vhas applied thereto a distant-crossing reference voltage from a source 144C, Vrepresenting thedistance Dbetween the track section between barriers X2 toi provide the required operating voltage, the relay ampliliern146C will operate relay 148C, thereby closing contacts 148CC. These, Yas shown in FIGURE 5A, are connected-to the relay 150'.

The contacts 150C of relay 150 can operate the warn ing device which is locatedv at-the'barrier crossing X3.

Computers C34, C23,-an d C12" are opera-ted when a train proceeds in the directionropposite to the one just considered. The .moder -of operation is identicalwith that described and therefore willnot be gone intoin detail. For example, for a train proceeding in the direction from the barrier X3 to the barrier X2, the computer C23 would Besides the relay amplifier 122B being operated when'the predicted time ofV arrival coincides with that which has been linserted in the computer,

the high-gain summing amplier 142D can operate the ,relay amplilier 146D when the sum of the voltages representingthe distance of the train from the barrier X2 and the length of Ithe track section D2 exceeds thevalue requiredto operate the relay amplifier 146D. Then the relay 148D is operated, in response to which its contacts 148DC are closed. This enables the energization of a ,relay 152, `which thereby closes its contacts 152C. These contacts are'connected in parallel'with the contacts 124C and therefore operate the warning device at the X1 barrier. Of course, where the track distances between crossings are sufficiently long, then the distance-crossing reference source and additional high-gain summing amplifier and relays need not be used.

The preceding analysis and discussion have assumed a rail condition such that the rails form a complete, homogeneous and uninterrupted circuit. However, one of the most annoying features fro-m the safety standpoint, and one over which there is little control, is the problem introduced by a badly bonded rail. Although all bonding may be good initially, there is always the possibility that the bond resistance may increase with time, either from corrosion or from mechanical loosening. If such a highresistance bond should come into existence, the total input impedance would rise and there would be an apparent increase in the distance from the crossing to the train.

If there is a high ballast resistance, the input impedance takes the form of zm=RL+jwlL (24) vwhere R==the effective A.C. resistance of the track per unit of length L=the length of track l=the effective A.C. inductance per unit of length W=21r times the excitation frequency In other Words, a linear relationship exists between Zin and L, between the total resistive component and L, and between the total reactive component and L. However, the presence of a high-resistance bond introduces a further factor, the Ibond resistance Rj, into the equation. This fact-or is independent of L, and thus a linear relationship no longer exists between Zh, and L, the degree of nonlinearity being a function of the magnitude of R5, the value of which can be rather large as in the case of .a broken bond wire without de-energizing a track relay to stop a train with a block signal.

The Ireactive portion of the input impedance is caused largely -by the fact that the transmission line is made yfrom a conductor of high permeability, and is unaffected by the bad bond. The linear relationship between Xe, the total reactive component, and L is thus still in existence. Hence, if the resistance component of fthe computer input signal is ignored and only the reactive ycomponent observed, the influence of a bad bond can be eliminated. A circuit f-or accomplishing this result is shown in FIGURES 6 and 7. This circuit is shown as a 4modification-of the embodiment of FIGURE 3. It should be obvious to those skilled in 'the art, however, that -similar modifications could be made to the embodiments of FIGURES 1 and 2.

Turning now to FIGURE 6, a train 10 is shown moving from left to right on tracks 12A and 12B. An excitation oscillator 170 drives a power amplifier 172. As was the case in the other embodiments, this oscillator may provide an output having a lfrequency on the order of 50* cycles per second. The output of this amplifier is applied to the vrails 12A, 12B; to point P directly and to point P through -a resistor 174. As before, the amplifier 172 and resistor 174 comprise a constant current generator. These points are also connected to a bandpass amplifier 176 whose frequency response is centered at the oscillator frequency and whose output is applied to a quadrature detector 178 which also has a reference input applied from the oscillator 176 through a phase shift network 179. In this modified circuit Ialthough a frequency of 50 cycles per second may be employed, a higher excitation frequency is preferably used, for example, a frequency of 265 cycles "per second and the bandpass amplifier 176 is provided with a wider band width so that it will have greater phase Ystability and enable the phase 'characteristics of the circuits ahead of the quadrature detector 178 to remain constant with time.

The output of the quadrature detector 178 is applied Vto an inverting dilerentiator circuit 180 and also to a summing .amplifier 182. The output of the inverting differentiator circuit is applied to an amplifier 184, the output of which is connected to a second input of the summing amplifier 18-2. The summing amplifier output is connected to a high-gain amplifier 186. The output of the high-gain amplifier is applied to an amplitude comparator 18S wherein it is compared with a signal from a reference vol-tage source 19t). The output of amplitude comparator 188 is connected to a relay amplifier 192 which operates the warning signal relay 194 when the signal applied thereto has a sufiicient amplitude.

An override circuit is als-o provided, and this includes an amplitude discriminator 198 which receives the output from the quadrature detector 178 and compares it to the output of a reference voltage source 198. The output of amplitude discriminator 196 is connected to a relay amplifier 200 which drives a minimum distance override relay y202.

As was the case with the circuit of FIGURE 3, the input to differentiator circuit `180 and summing amplifier 182 is a voltage proportional to the distance L between the train Iand the excitation points P and P. When differentiated, this vol-tage gives a voltage proportional to train speed. These voltages are utilized in the same manner as in FlGURE 3, the only difference being that the voltages depend only upon the reactive component of track impedance and not the resistive component. This will be apparent upon reference to FIGURE 7 which shows the quadrature detector 178 in greater detail, and to FIGURE 8 which shows various waveforms present inthe system.

In FIGURE 7 it can be seen that the reference input from the oscillator 170 is applied through the phase shift network 179 to the `input of a zero crossing detector 206. This zero crossing detector may, for example, comprise Ia Schmitt trigger .circuit 208 followed by a differentiator 210 and produces a trigger pulse for a pulse generator such as a mono-stable or one-shot multivibrator 212 when the reference voltage goes through zero with a positive slope. The output of the multivibrator 212 is a pulse of very short duration which is amplified by pulse amplifier 214 .and then applied to the gating input of a normally nou-conductive gate 216.

AThe signal input to the gate 216 is connected to the output of the bandpass amplifier 176.

When the gate 216 is turned on -by the generation of a pulse from the multivibrator 212, it forms a low impedance :charge or discharge path for a storage capacitor 21S in order that the capacitor -may assume the instantaneous value of the A.C. signal input. This stored voltage in the capacitor 21d is corrected once each cycle of the carrier frequency and will thus follow modulation of the carrier frequency. An impedance matching amplifier 220 is coupled to the storage capacitor 218 and is provided with a high input impedance to minimize loading of the capacitor, substantial unity gain, and low output impedance to drive the differentiator 180, summing amplifier 182 and amplitude discriminator 196.

The action of this quadrature detector can best be understood by reference to FIGURE 8. As can be seen from this figure, the A.C. signal input to the track, waveform C, can be considered as two components ladded together: the resistive component, waveform, CR, which is in phase with the A.C. reference voltage, waveform A; and the inductive component, waveform CX, which lags the reference by l degrees. Since the resistive component CR is always at zero when the gating pulse, waveform B, occurs, this component cannot `contribute to the stored voltage on the capacitor which is thus representative only of the peak value of the inductive component of AJC. signal voltage.

The phase shift network 179 is used to set a slightly negative net phase angle between the A.C. signal and the A.C. reference inputs to the quadrature detector with a resistor substituted for the track impedance. This -assured that'the phase angle will be zero or negative over a greater apparent distance signal with a correspondingly shorter warning time. This phase shift network may be inserted into the signal path rather than the reference path, if desired.

The modified cincuit of FIGURE 6 may, of course, be substituted for the embodiment of FIGURE 3 wherever the latter is used, for example, inthe compounded circuit of FIGURES 4, 5A and 5B.

formation for predicting the time required for arrival at a given location of a distant train which is moving on said tracks toward said location comprising means for applying an alternating-current signal with a constantcurrent level `to said tracks, means for deriving a voltage at said location, rectifier means to which said voltage is applied for providing a first output voltage having an instantaneous amplitude representative of the distance of sai-d train from said location and having the amplitude An addition-al benefit which may be obtained when this i invention is used is that of being able to detect when a bad bond between rails, or broken, or open rail occurs.

-This can be readily detected when the voltage being monitored at the crossing increases beyond the value normally obtained when no train is in the track section. If this voltage increases beyond the normal value, Ithen "there is indicated an increase in input impedance which is most likely caused by a broken rail or bad bond. This 'feature is usable with any of the embodiments of the invention or even independently, if desired.

Apparatus for detecting such detective rails is shown in FIGURE 5A for the D1 and D2 track sections. It

vincludes for respective track sections D1, D2 Ian Iamplitude discriminator 160A, 160B, respectively having one input connected to the output of the respective filters 108A, 198B, and -a second input connected respectively Vto the output of bad-bond reference-voltage sources 162A,

162B. The latter establishes the normal open-track section voltage reference and the `output of the filter 108A is the voltage actually existing -across the track section.

`When the voltage actually existing Iacross either the track natively, the output of either of the amplitude discriminatorsmay be employed to transfer the grade-crossing control back to the conventional block system until the defective rail is repaired.

Various :circuits have been represented herein by rectangles bearing the functional designation. These indi- Yvidual circuits are all well known in the electronic tield,

being amply described in the-literature and for the most part commercially purchasable as individual components from several manufacturers, either using vacuum tubes or transistors. Accordingly, detaileddescriptions of these circuits are omitted, since this would only add to the changing at a `rate representative of the speed of said train,-means for differentiating said first output voltage to obtain a second output voltage representative of said speed of said train, means for multiplying said second output voltage by a factor k to provide a third output voltage, andmeans to which said first and third output voltages are applied for obtaining a voltage representative of ther difference of said first and `third output voltages whereby said factor k represents the predicted time of arrival of said train at said given location when said difference voltage equals zero.

3. Apparatus yas recited in claim 2 wherein there is included means to apply the output of said means for obtaining a voltage representative of 4the difference of said first and third output voltages to said multiplying means to establish the factor k.

4. Apparatus for deriving from railroad tracks information for predicting the time required lfor arrival Y at a given location of a distant train which is moving on complexity of the description without. adding any further -1. Apparatus for deriving from railroad tracksinfor-v mation for predicting the time required for arrival at a Igiven location of a distant ltrain which is moving on said tracks toward said location :comprising means for applying an alternating-current signal with a constant-current level to said tracks at said location, means for deriving a first voltage at said location from said tracks which is Vrepresentative of the distance of said train from said location, means for differentiating said first voltage to 4obtain a second voltage representative of the instantaneous speed of said train, and means for combining said first and second voltages to provide a third voltage which is a function of the time required for said train to arrive at said given location, and means for utilizing said third voltage for operating a warning device at said given location. y

2. Apparatus for `deriving from railroad tracks insaid tracks toward said location comprising means for Vapplying, an alternating-current signal with a constantcurrent level to said tracks, means for deriving a voltage at said location, rectifier means to which said voltage is applied for providing a first output voltage having an instantaneous amplitude representative of the distance of said train from Vsaid location and having the amplitude changing at a rate representative of the speed Vof said train, means Yfor differentiating said first output voltage to obtain a second output voltage representative of said speed of said train, means for subtracting two voltages to provide a resultant voltage, means connected to the output of said means for subtracting for amplifying said -resultant voltage, means for multiplying the output of saidamplifying means by said second output voltage, and means for applying said first output voltage and the output of said means for multiplying to said means for subtracting two voltages to establish the output of said amplifying means as the predictedtime of train arrival.

5. Apparatus as recited in claim 4 wherein there is included means for establishing a reference voltage repre sentative of theY time interval desired before a train arrives at said given Ylocation for. operating a. warning device,

means to which said reference voltage and output from said amplifier` are applied for providing an output when these are equal, a warning device at said given location,

and means -for actuating said warning device responsive to said output.

6. Apparatus as recited in claim 5 wherein successive .lengths of said` track are bonded to each other and a defect in'said bonding may occur known as an open bond, there being included in said apparatus a means for establishing an open bond reference voltage equal in amplitude .to lthe output of said rectifier means when there is no tram on said tracks, amplitudeY comparing means `to which said open bond reference voltage and the voltage -output vof said rectifier means are applied for providing an out- 13 voltage at said location from said tracks which is representative of the distance of said -train from said location, means to which said first voltage is applied for differentiating said first voltage to obtain a second voltage representative of the instantaneous speed of said train,

a multiplying means having an output and a first and second input to which signals to be multiplied are applied, subtraction means to which the output lof said multiplying means and said first signal are applied for obtaining a third signal representing the difference therebetween, and means for applying said third signal and said second signal to said respective first -and second inputs of said multiplying means whereby said third signal represents the elapsed time at which said train will arrive at said given location measured from the instant said first voltage is derived.

8. Apparatus for deriving from railroad tracks information for predicting the time required for arrival at a given location of a distant train which is moving on said tracks toward said location comprising means for applying an alternating-current signal with a constantcurrent level to said tracks, means for deriving a first vol-tage at said location from said tracks which is representative of the ydistance of said -train from said location, means to which said first voltage is applied for differentiating said first voltage to obtain a second voltage representative of the instantaneous speed of said train, means to which said second voltage is applied for differentiating said second voltage and providing a third voltage representative of the acceleration of said train, rst, second, and third multiplying means each having two inputs to which voltages to be multiplied are applied and an output, means for applying said third voltage and said first multiplier output respectively -to said second multiplier two inputs, inverter means to which said first voltage is applied for inverting said first voltage, means connected to said inverter means output for doubling the amplitude of said inverted first voltage, means to which the output of said third multiplier means is applied for doubling said third multiplier Imeans output, summing means to which the outputs from said two means for doubling and said second multiplier means are applied for adding all said outputs and providing a fourth output voltage as the sum thereof, means for amplifying said fourth voltage, means for applying said second and amplified fourth voltages to the two inputs of said third multiplying means, and means for applying said amplified fourth voltage to the two inputs of said first multiplying means whereby said amplified fourth voltage represents the elapsed time required for said train to arrive at said location measured from the interval at which said first voltage is derived.

9. Apparatus for deriving from railroad tracks information for predicting 'the time required for arrival at a given location of a distant train which is moving on said tracks toward said location comprising means for applying an alternating-current signal with a constant-current level to said tracks, means for deriving a first voltage at said location from said tracks -which is representative of .the distance of said train from said location, means to which said first `voltage is applied for differentiating said first voltage to obtain a second voltage representative of the instantaneous speed of said train, means to which said second voltage is applied for multiplying said second voltage by a factor whose value equals a desired time interval be-fore said train arrives at said given location, means to which said first and multiplied second voltages are applied for subtracting said multiplied second voltages from said first voltage to provide a difference signal, means to which said difference signal is applied to detect when said difference signal is substantially zero at which time the time required for said `train to arrive at said given location equals said multiplication factor.

10. In a track system consisting of different track sections in series wherein there is a grade crossing separat- -ing adjoining track sections, apparatus Ifor providing a the distance of said train within that track section from said crossing, means for deriving from said first voltage a second Voltage representative of the velocity of said train within that track section, means for multiplying said second voltage by a factor k to provide a resultant voltage, means for establishing a third voltage representative 'of the length of the track section succeeding 4that in which said train is moving, means for combining said first voltage and said resultant Voltage, a first warning device for said grade crossing, means responsive .to said combined first voltage and-resultant voltage attaining a predeter- -mined value for actuating said first warning device,

means for combining said first voltage, said resultant voltage, and said third voltage, a second 'warning device for the grade crossing -between the succeeding two track sections, and means responsive to said combined rst resultant and third voltages attaining a predetermined value for actuating said second Warning device.

A11. 11n a track system as recited in claim 10 wherein successive lengths of said track are bonded to each other and a defect in -said bonding may occur known as an open bond, there is included for each track section means for deriving a voltage when no train is in said track section, means for establishing a bad-bond reference voltage having an amplitude substantially equal to that derived when no train is in said track section, amplitude cornparing means to which both said voltages are applied for providing yan ontputwhenever said yvoltage derived when no train is in said section exceeds said bad-bond reference voltage, and defective rail warning means to which said output is applied for actuating said defective rail warning means.

12. Apparatus for deriving from railroad tracks information for predicting the time required for arrival at a given location of a distant train which is moving on said tracks `toward said location comprising means for applying an alternating-current signal with a constant level to said tracks at said location, means for deriving a first voltage at said location from said tracks which is representative of the distance of said train from said location, said voltage having resistive and inductive components, means for modifying said Ifirst voltage by eliminating said resistive component from said first voltage, means Ifor differentiating said modified -first voltage to obtain a second voltage representative of the instantaneous speed of said train, and means for combining -said modified Voltage and said second voltage to provide a third voltage which is a function of the time required for said train to arrive at said given location, and means for utilizing said third voltage for operating a warning device at said location.

113. iAppara/tus for deriving from railroad tracks information for .predicting the time required for arrival at a given location of a dist-ant train which is moving on said tracks toward said location comprising means coupled to said tracks for applying an alternating current signal with a constant level to said tracks at said location, means coupled to said tracks for deriving a first voltage at said location from said tracks which is representative of the ldistance of said train from said location, said voltage having resistive and inductive components, means having a first input coupled to said deriving means and a second input coupled to a reference signal 'for modifying said first voltage by eliminating said resistive component from said first voltage, means coupled to said modifying means for differentiating said modified yfirst voltage to obtain a second voltage representative of the instantaneous speed of said train, and means coupled to said modifying means and said differentiating means for combiniug said mo-diiied rst voltage and'said second voltage to provide a third voltage which is a' function of the time required for-said train lto arrive at said given location, and means coupled to said combining means for said tracks towardsaid location comprising means for applying an alternating-current signal with a constant level to said tracks at said location, means for deriving a iirst voltage at said location from said tracks which is representative of the distance of said train from said location,

said voltage having -res-istive and inductive components, means for modifying said rst vol-tage by eliminating said resistive component therefrom, said means including gate means having a signal input coupled to Vsaid voltage deriving means for applying a gating signal to said gate means substantially only when the instantaneous value of said resistive component is zero, means for dilerentiating said modiiied first voltage to obtain a second voltage representative of the instantaneous speed of said train, and means for combining said modified vol-tage and said second voltage to provide a third voltage which is a function of the time required Vfor said train to arrive at Vsaid given location, and means for utilizing said third voltage for voperating a warning device at said location.

v a rst voltage at said location from said tracks which is ferentiatiug said modified iirst voltage to obtain a second fvoltage representative of the instantaneous speed of said train, and means for combining said modiied voltage and said second voltage to provide a third voltage which is a'function of the time required lfor said train to arrive at said given location, and means for utilizing said third voltage foi operating a warning device .at said location.

16, The apparatus of claim 15 wherein the output of said gate means is stored in storage means, said differentiating means being coupled to said storage means.

17. The apparatus of claim 15 wherein a small negative phase shift is-introduced between said alternatingcurrent signal and said derived voltage.

References Cited by the Examiner UNITED STATES PATENTS 7/1956 Whitmore 324-70 3/ 1960 Crawford 246-34 ARTHUR L. LA POINT, Primary Examiner.

LEO QUACKENBUSH, EUGENE G. BOTZ,

Examiners. 

1. APPARATUS FOR DERIVING FROM RAILROAD TRACKS INFORMATION FOR PREDICTING THE TIME REQUIRED FOR ARRIVAL AT A GIVEN LOCATION OF A DISTANT TRAIN WHICH IS MOVING ON SAID TRACKS TOWARD SAID LOCATION COMPRISING MEANS FOR APPLYING AN ALTERNATING-CURRENT SIGNAL WITH A CONSTANT-CURRENT LEVEL TO SAID TRACKS AT SAID LOCATION, MEANS FOR DERIVING A FIRST VOLTAGE AT SAID LOCATION FROM SAID TRACKS WHICH IS REPRESENTATIVE OF THE DISTANCE OF SAID TRAIN FROM SAID LOCATION, MEANS FOR DIFFERENTIATING SAID FIRST VOLTAGE TO OBTAIN A SECOND VOLTAGE REPRESENTATIVE OF THE INSTANTANEOUS SPEED OF SAID TRAIN, AND MEANS FOR COMBINING SAID 